A rubber crawler is driven by either engaging projections on an inner peripheral surface of the rubber crawler with sprockets (coreless type drive system) or engaging cores embedded in the rubber crawler with sprockets (core type drive system).
FIGS. 1A, 1B and 2 each show a conventional drive wheel 20. FIG. 1A shows a structure in which a pair of discs 21 and 22 fixed to a drive shaft face each other and a predetermined number of pins 23 extend between the circumferences of the discs 21 and 22. Further, FIG. 1B shows a structure in which annular flanges 24 are formed at the peripheries of the discs 21 and 22. Furthermore, FIG. 2 shows the structure of a star-shaped drive wheel which does not have the discs described above but has a predetermined number of teeth 26 on the periphery of a hub 25 fixed to a drive shaft.
When these drive wheels are used for, for example, the coreless type rubber crawler, the pins 23 and the teeth 26 engage with the projections formed on the inner peripheral surface of the rubber crawler at predetermined intervals, such that the pins 23 and the teeth 26 can successively move together with the projections and transmit a driving force to the rubber crawler.
Because of this structure, soil and fibrous materials cannot be prevented from entering the inner periphery of the rubber crawler during the running (or operation) thereof. When soil is caught up between the rubber crawler and the sprocket (or an idler wheel), phenomena occur such as inadequate engagement of the rubber crawler with the sprocket and application of excessive tension to the rubber crawler. Due to these phenomena, the rubber crawler may be disengaged from the sprocket, and consequently, the circumference of the rubber crawler may deviate, thereby making the rubber crawler unusable.
More specifically, the drive wheels shown in FIGS. 1A and 1B are structured such that the running-off (or disengagement) of the rubber crawler is prevented by pressing the protrusions of the rubber crawler from the right and left by the discs 21 and 22, since the discs 21 and 22 are present at the right and left. Moreover, in FIG. 1B, the flanges 24 contact the inner peripheral surface of the rubber crawler so as to support the tension applied thereto.
However, a drawback arises in that the rubber crawler does not fully engage with the drive wheel since fibrous materials such as grass and straw easily get tangled around the pins 23. Further, the vicinity of the rubber projections of the rubber crawler (right and left sides) is closed by the discs 21 and 22, thereby forming a closed space. With this structure, when the rubber projections engage with the drive wheel, mud and the fibrous materials around the rubber projections become hard and build up without escaping to the outside. This buildup layer gradually becomes thick during running, resulting in an increased gap between the inner peripheral surface of the rubber crawler and the drive wheel. Thus, the tension applied to the rubber crawler becomes extremely large, and steel cords embedded in the rubber crawler remain fully stretched, thereby shortening the life of the rubber crawler. The rubber crawler is inevitably disengaged from the drive wheel due to their insufficient engagement.
In contrast, in the drive wheel shown in FIG. 2 has no discs (21, 22) at the right and left sides of engaging teeth which engage with the rubber projections. Thus, the engaging teeth are exposed, and few fibrous materials get tangled around the engaging teeth 26. Further, the fibrous materials and soil are easily discharged, and the phenomenon of soil and the like near the rubber projections becoming hard hardly occurs. As a result, the gap between the inner peripheral surface of the rubber crawler and the drive wheel is hardly increased, and the tension applied to the rubber crawler does not become extremely large. However, since the discs 21 and 22 are absent, the drive wheel has no function of preventing lateral displacement of the drive wheel and the rubber crawler relative to each other. For this reason, running off of the rubber crawler from the drive wheel is inevitable when the drive wheel is rotated or an external force is applied to the rubber crawler in a lateral direction.